FIG. 6 is an enlarged view of a partial cross section illustrating an entire configuration of a boiling water reactor. FIG. 7 is an enlarged sectional view of a reactor bottom portion shown in FIG. 6.
As shown in FIGS. 6 and 7, a CRD housing 3 for storing a CRD and a stub tube 2 for supporting the CRD housing 3 are provided for a bottom head 1 of a reactor pressure vessel in a boiling water reactor.
As illustrated in FIG. 7, the stub tube 2 is welded to a reactor pressure vessel bottom head (or lower-side end plate) (hereinafter simply referred to as a “bottom head”) 1 by means of a welded portion 4. The CRD housing 3 passing through the stub tube 2 and the bottom head 1 is welded to an upper portion of the stub tube 2.
Further, as shown in circular portions “a”, “b” and “c” of FIG. 7, sensitivity to stress corrosion cracking becomes high particularly in a heat affected portion in a vicinity of the welded portion. It has been reported that in older plants built before knowledge of stress corrosion cracking was available, cracking may occur due to a fault inherent in the welded portion, and as a result reactor water leaks through the cracking.
In a conventional method, when cracking occurs in a stub tube in a penetrating portion of a CRD housing in the reactor bottom portion and a reactor water leakage is detected, a method or treatment replaces an entire CRD housing and stub tube or expands and deforms the tube of the CRD housing from inside by a roller to be pressed against a through hole of the bottom head of the pressure vessel to plug the leakage route. Alternatively, the area with a leak can be enclosed with a cylindrical mechanical seal member.
As the repairing method in a case of a shallow and non-penetrating crack, a method in which a cracked portion is removed by mechanical processing or electrical-discharge machining and then overlay repairing is performed by TIG welding or laser welding, or a welding method for welding only a surface crack, has been studied.
The conventional method of replacing the entire CRD housing and stub tube requires a long time for preparatory and replacement work, resulting in a long time needed for stopping of the operation of the reactor, and hence, highly increasing working cost.
Although the roller tube expansion method is a simple and easy method, re-leakage from a tube expansion portion is likely to occur due to thermal deformation in the operating cycle of the reactor and thus, complete sealing for a long period of time is difficult.
Further, the seal method of attaching a sealing mechanism member is a method only applicable to a limited leak position.
According to the conventional methods (see Patent Documents 1 to 6) in which the cracked portion is removed by mechanical processing or electrical-discharge machining and then overlay repairing is performed by TIG welding or laser welding, there is a high possibility that a new crack occurs due to a flaw of an existing stub tube or a welded portion thereof. It is difficult to perform a particular treatment on this portion and thus the conventional methods are not permanent measures.
Furthermore, in the conventional technology, generation of a crack and the repairing thereof in a cylindrical body such as a penetrating tube for a reactor core measuring sensor provided in the reactor bottom portion of the pressurized-water reactor constitute a significant matter to be solved.
Patent Document 1: Japanese Patent Laid-Open No. H10-030991
Patent Document 2: Japanese Patent Laid-Open No. 2003-320472
Patent Document 3: Japanese Patent Laid-Open No. 2004-226329
Patent Document 4: Japanese Patent Laid-Open No. 2004-294372
Patent Document 5: Japanese Patent Laid-Open No. 2006-337175
Patent Document 6: Japanese Patent Laid-Open No. 2008-020447